Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are generally used as a solvent for dissolving an electrolyte salt for lithium secondary battery. However since these hydrocarbon carbonates are low in a flash point and have high combustibility, there is a danger of firing and explosion by over-charging and over-heating, which is an important problem to be solved for securing safety especially in the cases of large size lithium secondary batteries for hybrid cars and distributed power source.
For preventing explosion of an electrolytic solution, means for blending fluoroalkane, phosphoric ester or phosphorus compound as an additive to the electrolytic solution are proposed (JP11-233141A, JP11-283669A, JP2002-280061A and JP9-293533A).
However, in a system where fluoroalkane is added, fluoroalkane itself is hardly compatible with carbonates being essential as components of an electrolytic solution, thereby causing phase separation and deteriorating battery performance.
Also, in a system where phosphoric ester or phosphorus compound is added, combustibility of an electrolytic solution is inhibited, but viscosity becomes high, thereby easily decreasing conductivity and easily causing deterioration due to charge and discharge cycle.
In order to improve noncombustibility and flame retardancy of an electrolytic solution without lowering its performance, addition of a fluorine-containing ether has been proposed (JP8-37024A, JP9-97627A, JP11-26015A, JP2000-294281A, JP2001-52737A and JP11-307123A).
JP8-37024A describes an electrolytic solution for secondary battery comprising a fluorine-containing ether and having high capacity and excellent cycle stability, and says that either of chain fluorine-containing ether and cyclic fluorine-containing ether may be used, and fluorine-containing ethers having an alkyl group having 2 or less carbon atoms at one end thereof are exemplified as examples of chain fluorine-containing ether.
However, it is disclosed that the content of fluorine-containing ether is up to 30% by volume, and when the content is larger than 30% by volume, in such a system, discharge capacity becomes small.
In order to prepare an electrolytic solution without using cyclic carbonate as a solvent for dissolving an electrolyte salt, JP9-97627A proposes using, in addition to non-cyclic carbonate, a fluorine-containing ether represented by R1—O—R2 (R1 is an alkyl group or halogen-substituted alkyl group having 2 or less carbon atoms; R2 is a halogen-substituted alkyl group having not less than two and not more than 10 carbon atoms) in an amount of more than 30% by volume and not more than 90% by volume. Also it is suggested that initial discharge capacity is improved by blending cyclic carbonate preferably in an amount of not more than 30% by volume, though blending of cyclic carbonate is not essential.
However, JP9-97627A says that in this system, when the number of carbon atoms of R1 is 3 or more, solubility of an electrolyte salt is lowered, and target battery characteristics cannot be obtained.
JP11-26015A, JP2000-294281A and JP2001-52737A propose improvement in compatibility with other solvent, stability for oxidation decomposition and noncombustibility by using a fluorine-containing ether having —CH2—O— as an organic group having ether linkage-formable oxygen, and concretely disclose a fluorine-containing ether such as HCF2CF2CH2OCF2CF2H having an organic group having 2 or less carbon atoms and being bonded to one end of the ether linkage-formable oxygen. However, generally its boiling point is low, compatibility with other solvent is low and in addition, solubility of an electrolyte salt is low. Therefore, this fluorine-containing ether is not necessarily enough as a solvent for an electrolytic solution for secondary battery in the case of aiming at further heat resistance and resistance to oxidation.
JP11-307123A describes that an electrolytic solution being excellent in keeping of capacity and safety can be provided by mixing a fluorine-containing ether represented by CmF2m+1—O—CnH2n+1 to chain carbonate. However, this solvent mixture system is low in capability of dissolving an electrolyte salt and cannot dissolve LiPF6 and LiBF4 which are excellent electrolyte salts and are generally used. As a result, LiN(O2SCF3)2 exhibiting corrosive behavior on metal is obliged to be used as an electrolyte salt. Also, rate characteristics are inferior because of high viscosity.
As mentioned above, the present situation is such that electrolytic solutions for lithium secondary battery being excellent in noncombustibility and flame retardancy and having sufficient battery characteristics (charge and discharge cycle characteristics, discharge capacity, etc.) have not been developed.